(i) Field of the Invention
This invention relates to an improved hydrometallurgical process for treating nickel-copper mattes for the recovery of copper and nickel values. More particularly, the invention involves a process for treating iron and arsenic containing nickel-copper matte to thereby produce an essentially copper-free solution of nickel sulphate, from which metallic nickel can be recovered by processes such as electrowinning, and a copper sulphide concentrate containing low levels of iron, arsenic and nickel, from which copper can be recovered by a variety of conventional processes.
(ii) Description of the Related Art
Copper is recovered from copper sulphide concentrates by a variety of industrial processes including smelting and electrorefining, and roasting, leaching and electrowinning. In all cases it is desirable to ensure that nickel and arsenic, when present in the ore or concentrate, are separated from the copper prior to the electrolytic process step, in which they dissolve and accumulate in the electrolyte, with deleterious effect on cathode product quality and process economics. In the roast-leach-electrowinning process it is also desirable to minimize the level of iron in the copper sulphide concentrate to prevent the formation of acid-insoluble copper ferrites in the roasting step, which can significantly reduce the copper recovery attainable by this process route.
Copper frequently occurs in combination with nickel in sulphide ores, with the most common minerals being chalcopyrite, CuFeS.sub.2, and pentlandite, NiFeS.sub.2, in which the copper and nickel occur in combination with iron and sulphur. Arsenic is frequently present in low concentrations in nickel-copper sulphide ores. The conventional method for recovering nickel and copper from these ores is by flotation and smelting to produce a nickel-copper matte, typically containing 70 to 94% Ni+Cu, 5 to 22% S, and 0.2 to 5% Fe. Such mattes may also contain up to 5% arsenic, with lesser amounts of antimony and bismuth.
One well established industrial process for the recovery of nickel from nickel-copper mattes utilizes oxidative sulphuric acid leaching at atmospheric pressure to selectively leach nickel to form a relatively pure nickel sulphate solution, which after final purification, forms the feed to a nickel electrowinning process. In this process the copper is recovered as copper sulphide in the leach residue, but this residue also contains substantial amounts of nickel and significant amounts of iron, and in some cases, arsenic. Such a residue typically contains 45 to 55% Cu, 10 to 20% Ni, and up to 5% Fe and As. Before this copper sulphide residue can be processed by the conventional processes to copper metal, the levels of nickel, iron and arsenic must be reduced to less than about 1%.
A number of processes have been proposed to remove nickel, or nickel and arsenic, or nickel and iron from such copper sulphide containing leach residues, but none has yet succeeded in providing a process to remove all three of the metal contaminants. Prior art leach processes illustrative of the art are disclosed in U.S. Pat. No. 5,344,479 granted Sep. 6, 1994 to Sherritt Gordon Limited. PCT published application No. PCT/FI96/00432 filed Aug. 6, 1996, the assignees being Outokumpu Engineering Oy and U.S. Pat. No. 5,628,817 granted May 13, 1997 to Outokumpu Engineering Oy. Also of note, is U.S. Pat. No. 3,616,331 granted Oct. 26, 1971 to the International Nickel Company Inc. and U.S. Pat. No. 4,323,541 granted Apr. 6, 1982 to Outokumpu Engineering Oy.
In U.S. Pat. No. 5,344,479 issued to Kerfoot et al. there is disclosed a process for leaching a finely divided nickel-copper matte in acid solution under oxidizing conditions in an atmospheric leach step to produce a nickel sulphate solution and a copper-rich sulphide residue. The copper-rich sulphide residue is separated from the nickel sulphate solution and pressure leached under a non-oxidizing atmosphere in a sulphuric acid solution to produce a nickel sulphate solution containing iron and arsenic and a low-nickel copper sulphide product essentially free of iron and arsenic. The nickel sulphate solution is then treated in an iron-arsenic precipitation step in which the iron and arsenic are precipitated as ferric arsenate. The ferric arsenate precipitate is separated from the nickel sulphate solution and discarded. The nickel sulphate solution is recycled into the atmospheric leach step.
PCT application No. PCT/FI96/00432 discloses a modified process for recovering nickel and copper and separating iron from two pyrometallurgically produced nickel mattes containing different amounts of iron. The matte containing the lower amount of iron is leached in acid solution under oxidizing conditions at atmospheric pressure to selectively leach nickel from the matte to produce a nickel sulphate solution and a copper-rich sulphide residue. The copper-rich sulphide residue is separated from the nickel sulphate solution and pressure leached under a mildly oxidizing atmosphere in an acid solution to produce a low nickel copper sulphide product. The matte containing the higher amount of iron is treated in the solution from the pressure leach step in a combined oxidative atmospheric leach and iron hydrolysis step in which the solution pH is adjusted to at least 1.0 or higher. The nickel content of the high-iron matte is leached quantitatively, and iron is precipitated as a jarosite or goethite. The low-iron nickel sulphate solution is recycled back into the atmospheric leaching step.
U.S. Pat. No. 5,628,817 discloses a process for leaching nickel and copper from a high-sulphur low-iron nickel-copper matte by means of a multi-stage process. The nickel copper matte is leached in two atmospheric pressure leaching steps in the presence of oxygen using neutral or acidic leaching solutions containing copper sulphate, to produce a nickel sulphate solution and a precipitate of nickel and copper sulphides. The nickel is recovered from the nickel sulphate solution by electrowinning. The nickel and copper sulphide precipitate is then pressure leached in a substantially neutral copper sulphate solution, to produce a solution of nickel sulphate and a copper-rich precipitate. The iron in the nickel sulphate leach solution is then precipitated in an iron removal step and the residue proceeds to an oxidative pressure leach step in which the copper sulphide is quantitatively dissolved. The iron-free nickel sulphate solution from the iron removal step is recycled to the atmospheric leach circuits.
The commonality between these prior art processes resides in the fact that the iron and arsenic removal step is conducted following the non-oxidizing or mildly oxidizing pressure leach.